Doppler Effect

Jonathan Osbourne

The Doppler effect is when either the source or observer of a wave in relative motion. The wave is perceived differently by an observer in motion and appears to be squished in front of the path of the observer and stretched in behind it. Redshift is the term used for the increase in the wavelength of light that is moving away. Red has lower wavelength than other colors of light. Blueshift is the term used for light moving towards the observer because its wavelength increases.

So let's talk about the Doppler effect. What is the Doppler effect? The Doppler effect is something that happens when you've got either a source of a wave or an observer of a wave or both in relative motion.

So I could have for example an ambulance with a siren and it's moving towards me. So that means, that the wave that it's creating is going to be perceived differently by me than it would be if the ambulance was just sitting there parked, playing its siren. Alright. So, what happens?

The wave will be squished in front and it will be stretched in back. Alright? So that means that when the source and the observer are moving towards each other, the wave is squished. So that means that the wavelength is smaller and the frequency is larger than it would be if there wasn't any relative motion. Alright? On the other hand, when the two objects are moving away from each other, then the wave is going to be stretched. So the wavelength is larger and the frequency is smaller. And let's see a demonstration of this.

So, as you can see the car starts, here it is at rest and now he's going to move and look at that, squished in front, stretched in back. It's a standard easy, very very very simple thing to understand when you look at how it works. It's moving, it squishes the wave. Wavelength is shorter, that means the frequency is bigger. It's moving away, it stretches out the wave. That means the wavelength is longer, the frequency is smaller. Alright. Let's do some example problems involving this. Now I'm not going to really talk about this quantitatively, like how could you calculate the frequency. There are of course formulas for that but most introductory Physics courses don't require that you actually calculate that stuff.

So number 1. A car's moving towards you and it speeds up. Now, if it's honking its horn while it speeds up, what will happen to the frequency that you're perceiving? Well, it's moving towards you but it's speeding up. So that means that it's going to squish the wave front even more so that means that the frequency will get higher. So it will go like that. Alright.

So what about number 2? A star is moving away from the earth, alright? Now it's moving away. Now, what's the wave that we're perceiving from the star? The light wave. This also works with light. It works basically with any type of wave. So the star's moving away from us and so that means that the wavelength is going to be bigger. Alright. Now, when we think about visible light, we think 400 nanometers to 700 nanometers. 400 is violet 700 is red light. So when things have a larger wavelength, we say that they're red shifted. So this light will be red shifted. Red shifted means moving away. Alright?

Now we've got an example where a space ship is moving towards a star and it speeds up, alright? So it's going towards, that means it's squishing the wavelength, it's making it smaller. So we might want to call this violet shifting. But for some reason we don't. We call it blue shift. So with light, these are two terms that you'll see. Red shift, moving towards oh sorry. Moving away. Blue shift, moving towards.